For IC deposition, printer platforms are supplanting dispensers on die bonders.

I have previously talked about the inherent advantages of mass imaging techniques when applied to semiconductor packaging. The same efficiencies - speed, accuracy, limitless deposit volumes - that have been realized in the surface-mount market have also taken hold in packaging. Some of the countless examples of printing applications within semiconductor processing include wafer bumping, solder ball placement and encapsulation. Advancements in wafer handling and advanced materials deposition techniques have taken the crossover technology advantages of the printing platform and enabled it for ultra-thin die attach materials applications, moving the process closer to mainstream manufacturing.

Traditionally, die attach adhesive pastes have been applied with a conventional dispensing system as part of the die bonding machine. But this presents challenges as semiconductor manufacturers move to higher volume requirements and strive to reduce device footprints. Just as in the SMT market, placement speed has increased dramatically in packaging. With dispensers, the only way to approach the volumes now being produced with modern placement systems is to add more dispensers, which is prohibitive in terms of cost and floor space.

Dispensing also has drawbacks for the chip footprint because the process requires the formation of an adhesive fillet around the chip, which increases the overall area of the chip. Also contributing to a larger footprint is the tendency of paste adhesives for resin bleed. To accommodate for this, space has to be left around the surrounding bond pads to ensure that they are not affected by excessive bleed. Not only are there speed and size constraints with conventional die attach methods, but there are also quality and reliability concerns. Accurately placing a chip into a tiny dot of adhesive is a highly delicate operation. If there is any tilt to the chip, or if the chip is applied with too much or too little force, there are problems with even distribution of the die attach material. If the adhesive coverage is insufficient or uneven, reliability of the final package is in question.

A mass imaging/printing platform for die attach material deposition can resolve many dispensing-related issues. Using a printing platform, new developments in wafer handling and materials application techniques have resulted in the ability to coat wafers with ultra-thin die attach materials down to 25 µm thickness with speed, accuracy and uniformity. One of the most delicate aspects of wafer printing is providing the stability to coat an extremely thin - 50 µm - yet, relatively large in diameter - 300 mm - wafer. To ensure uniform material deposition, the wafer must be stable, secure and perfectly flat during the imaging process. An ultra-flat pallet is the key enabler here (Figures 1, 2 and 3). The securing mechanisms of some pallets tend to pull the wafer so that either a) the wafer can break from the force or b) the wafer is not perfectly flat, which leads to uneven distribution of the material being applied. Also key: the precision of the printer and the tool used to apply the adhesive paste. We suggest a specially engineered squeegee in combination with a die attach stencil or screen.

The advantages of this process are the elimination of the fillet and subsequent reduction of paste adhesive volume - potentially by a factor of 10 on die less than 0.5 x 0.5 mm, the ability to print material in complex shapes without any adverse effect on line speed and simplifying the supply chain by requiring only one paste formulation for various process needs. Mass imaging of adhesive paste for die attach also has cost savings over dry film. Film can cost as much as 30% more than adhesive paste and has to be manufactured in multiple thicknesses and widths for different applications so is not as versatile as paste.

Then there is the ability to process even thinner wafers. We have observed the coating of 150 mm wafers only 50 µm thick. In our estimation, backside wafer coatings for die attach and other applications such as epoxy and wafer-level protective coatings will emerge as the standard for semiconductor processing.

 

Clive Ashmore is global applied process engineering manager at DEK (dek.com); cashmore@dek.com. His column appears semimonthly.